Control method and device for a display device

ABSTRACT

A display device ( 1 ) comprising: a liquid crystal display ( 2 ) having pixels ( 8 ) which are arranged in columns C and rows R; and a gate driving arrangement ( 4 ) for activating the pixels ( 8 ) of rows (R) in dependence on a row scanning scheme; a source driving arrangement ( 3 ) for providing column voltages representing the respective image data, a number of buffers ( 40 ) depending on the number of grey levels for supplying the respective voltage levels to the columns (C), a switching unit ( 31 ) for selecting a voltage level which corresponds to the grey level to be displayed; and a control unit ( 32 ) for controlling the buffers ( 40 ). For reducing the power consumption, an analyzing unit ( 39 ) is provided which analyzes the image data to be displayed such that the control unit ( 32 ) can determine the number of required buffers in dependence on the number of grey levels to be displayed and can switch off any unused buffers.

The present invention generally relates to a liquid crystal displayhaving pixels which are arranged in columns and rows. In particular, thepresent invention relates to an LCD device with a gate driverarrangement for activating the pixels of rows in dependence on a rowscanning scheme; and a source driver arrangement for providing columnsvoltages representing the respective image data, and a number of buffersfor supplying the respective voltage level in dependence on the numberof grey levels to be displayed, and a switching unit for combining theimage data with the voltage level; and a control unit for controllingthe buffers. The invention also relates to a method of driving a displaydevice.

Liquid crystal display devices have a wide range of applications, i.e.for mobile phones, personal digital assistants, notebooks, or TVscreens.

There are two kinds of displays, on the one hand passive matrix displaysand on the other active matrix displays (AMLCD), which are also calledTFT-displays. The present invention relates to TFT-displays, which areoften used in portable appliances, so it is particularly important torealize a low power consumption. Many applications of display devicesare battery operated, most being mobile phones. Battery lifetime is oneof the key market drivers for such phones. If the power consumption ofsuch a device can be reduced, the standby time can be increased.Alternatively, the battery capacity may be reduced giving a reduction inweight, another key factor.

An active matrix display (AMLCD) driving arrangement can be divided intoa gate or row driver and a source or column driver, which two parts maybe on a single chip or split up into two chips. The gate driver controlsthe gates of the thin film transistors (TFT) of the pixels by switchingon or off all TFTs of a specific selected row. Each pixel of a displayis composed of three sub-pixels presenting the colors red, green andblue by using filters. Each pixel has its own storage capacitor, whichstores the charge on this pixel for one frame. When a specific row isactivated or selected by the gate driver, the source driver provides toeach sub-pixel of this row the required column voltage, corresponding tothe grey level to be displayed of the respective sub-pixel. The humaneye makes a mixture of the grey levels of the sub-pixels to obtain thefinal color. When the scanning scheme continues with the following row,the previous row is deactivated by switching-off of the gates of theTFTs in that row and the charge value is kept by the storage capacitor.

To generate grey levels, a resistor chain may be used which divides thesupply voltage into several partial voltages. Each partial voltage isbuffered by a buffer to generate voltage levels. There are as manypartial voltages and buffers as grey levels are required. These severalvoltage levels are supplied to a switching unit. The image data are alsosupplied to the switching unit. The switching unit selects a voltagelevel, which corresponds to the grey level to be displayed. Thus thecolumn voltage is generated, which is supplied to the respective columnoutput pad of the display. The source driver supplies a respectivecolumn voltage to each column. The number of different column voltagesdepends on the number of grey levels to be displayed.

It is known to change the amount of colors for certain operationalmodes. Thus only a few colors are driven or a black and white mode isused in the standby mode, but in the video mode all possible grey levelsare driven to display all possible colors.

It is accordingly an object of the invention to provide a display devicehaving a further reduced power consumption, especially a powerconsumption which can be reduced independently of the operational modeused.

This object is achieved by the features of claim 1.

The invention is based on the idea that most displays can be operated indifferent modes. The driving arrangement for providing the requiredcolumn and row voltages is capable of driving each mode, such that partsof the driving arrangement are only needed for a certain mode. Thiscauses an unnecessary power consumption in the respective unused parts.This means that not all buffers are required for driving only a smallamount of colors in certain modes. By analyzing the number of greylevels in the image data to be displayed it is possible to drive onlythe required buffers and to switch off all unused buffers. Especiallymobile phones are mostly used in a still picture mode or a standby mode,because no video mode is required for occasional phoning. The mobilephone, however, should be capable of displaying videos. The use of theinvention can save a large amount of power.

To achieve this, a control unit is arranged in the source driver, whichunit analyses the supplied image data. Depending on the grey levelswhich should be displayed, the number of buffers is determined by thecontrol unit. The unused buffers are switched off by the control unit.

One aspect of the present invention leads to an embodiment having amemory unit incorporated in the source driver. It is advantageousespecially for a still picture mode and a standby mode to avoid aconstant transmission of image data for pictures which are notconstantly changing. These image data can be stored in the memory unit.

In the video mode, the picture to be displayed is constantly changing,so it makes no sense to store the image data in the memory unit, becausethe memory has to be written and read out at least as much as thepicture changes. In this video mode the memory unit is bypassed and thedata are directly transmitted through an interface from an externalstorage medium, a recording or playback-device.

Another aspect of the present invention results in a display device, inwhich the control unit is provided for analyzing the memory content whenthe memory is written, the number of required buffers is determined independence on the grey levels contained in the written image data, andthe buffers not required are switched off by the control unit. Thememory is only rewritten if the image data has changed, so in themeantime there is no change of the grey levels to be displayed.Therefore, only the buffers for driving the voltage levels for theseused grey levels are switched on. The remaining buffers are switched offby the control unit until the memory is rewritten. The image data areanalyzed before they are written in the memory, which can be performedvery easily, because the control unit controls the writing process tothe memory. So the image data can be analyzed before they are suppliedto the memory.

This embodiment has the advantage that the additional routing effort isvery small. The analyzing part in the control unit does not need extraspace, because the analysis can be done by existing circuitry.

A further aspect of the present invention yields a display devicewherein the control unit is provided for analyzing the memory contentwhen the memory unit is read out, the number of required buffers isdetermined in dependence on the grey levels of the read out image datato be displayed, and the buffers not required are switched off by thecontrol unit. In this embodiment the image data are analyzed after theyhave been stored in the memory. So there has to be a feedback line fromthe memory output to the control unit. This has the advantage that thereis more time for analyzing the image data, because the analysis can bedone while the picture is being displayed.

A further embodiment of the present invention relates to a displaydevice having feedback lines between the buffers and the control unit.After a first frame of rows has been scanned, it can thus be checkedwhich buffers have supplied a current to the switching unit. If thebuffer has not supplied a current to the switching unit, it was notused, which means the grey level which corresponds to that voltagelevel, which is driven by that buffer, was not contained in the imagedata. So this buffer can be switched off. All unused buffers remainswitched off until the respective memory content changes.

The ON/OFF switching of the buffers is realized in all cases in thatcontrol lines are provided between the buffers and the control unit.

In a further embodiment, the on/off switching of the buffers can belinked to the used operational mode, i.e. the on/off switching ofbuffers is only possible in the standby or in the still picture mode.

The object of that invention is also solved by a method of driving adisplay device having pixels arranged in columns and rows, comprisingthe following steps: the number of used grey levels for the image to bedisplayed is analyzed; the number of buffers for providing the columnvoltages is determined in dependence on the number of grey levels used,and the unused buffers are switched off.

In order that the invention may be well understood, some embodimentsthereof will now be described by way of example, reference being made tothe accompanying drawings, in which:

FIG. 1 shows a block diagram of a display device according to thepresent invention;

FIG. 2 shows a block diagram of a display device, where the image dataare analyzed before being written into the memory according to thepresent invention;

FIG. 3 shows a block diagram of a display device, where the image dataare analyzed after reading out of the memory according to the presentinvention;

FIG. 4 shows a block diagram of a display device, where the buffers areanalyzed after a first scan of the rows according to the presentinvention;

FIG. 1 shows block diagram of a display device according to theinvention. The display device 1 comprises an LCD panel 2 having n rows Rand m columns C. A gate driving arrangement 4 provides the respectiverow or selection voltages over the gate lines 6 in dependence on ascanning scheme. The source driving arrangement 3 provides the columnvoltages to the columns C of the LCD panel 2 over the source lines 7.The source driver 3 comprises a voltage generation unit 33 in which therequired voltages levels are generated. These voltage levels areprovided to a switching unit 31. The source driver 3 further comprises acontrol unit 32 which receives the image data via line 35. The imagedata are provided by a device (not shown), for example a graphiccontroller or the like. The control unit 32 is connected to the voltagegeneration unit 33 for controlling it. Furthermore, it is arranged forproviding the image data to the switching unit 31. In the switching unit31 the voltage levels are assigned to the supplied image data forproviding the respective column voltages to the columns C. The controlunit 31 analyzes the supplied image data and determines the number ofgrey levels in the image data to be displayed. The control unit 32 isable to switch off all unused buffers in the voltage generation unit 33on the basis of the information how many and which grey levels arerequired for showing the image data on the LCD-panel 2. A large amountof power can be saved in this manner.

FIG. 2 shows a block diagram of a display device 1, where the image dataare analyzed before being written into the memory unit 37 according tothe present invention. The voltage generation unit is comprised of aresistor chain 38 and buffers 40, which operate as follows. The partialvoltages may also be generated by a capacitor divider. The generatedvoltage levels are supplied to a switching unit 31. The control unit 32contains an analyzing unit 39. In this embodiment a memory unit 37 isarranged for storing the image data until the image is changing. Theimage data is supplied to the control unit 32 and the control unit 32transmits the image data to the memory unit 37. Each time the memoryunit 37 is written, the analyzing unit 39 analyzes the image data 35,which are written into the memory unit 37 via a feedback line 42. Thestored image data in the memory unit 37 are supplied to the switchingunit 31, which selects for each column C the respective voltage levelcorresponding to the image data. In this way the column voltages aregenerated and supplied to the column output pads 36, which are connectedto the column line 7 of the LCD panel 2. By analyzing the image databefore they are written into the memory, the analyzing unit determinesthe number of required buffers. The buffers for grey levels which arenot included in the image data are switched off by the control unit 32via the switching lines 41. They remain switched off until the memoryunit 37 is rewritten.

FIG. 3 shows a block diagram of a display device 1, where the image dataare analyzed after reading out of the memory unit 37 according to thepresent invention. There is a feedback line 43 coupled from the outputof the memory unit 37 to the analyzing unit in the control unit. Thecontrol unit 32 switches off all buffers 40 that are not required. Theunused buffers remain switched off until the image data change.

FIG. 4 shows a block diagram of a display device 1, where the buffersare analyzed after a first scan of the rows R according to the presentinvention. Each buffer has a feedback line 44 coupled to the controlunit. After a first frame of rows has been scanned, the analyzing unit39 analyzes whether the buffer has supplied a signal, i.e. current, tothe switching unit 31. If a buffer 40 has not supplied a signal, therespective buffer was not used and may therefore be switched off via therespective switching line 41.

The analyzing unit 39 may be realized as a counter, which counts thecoded grey levels contained in the image data. The counter may berealized with existing circuitry in the control unit 31.

In the following an example will be explained. An LCD module of a mobilephone is capable of showing e.g. 4000 colors, i.e. 32 buffers arerequired. As long as the mobile is in the standby or the still picturemode, only the network provider is displayed or information from theaddress book or the telephone number of an incoming call, etc. Thismeans that only a small amount of colors is needed, which requires asmaller number of buffers, so the rest of the buffers can be switchedoff. The number of colors needed will be analyzed within the controlunit 32 during the writing process or the read out process or after thefirst scan of rows with the feedback lines 44.

1. A display device comprising: a liquid crystal display shaving pixelswhich are arranged in columns C and rows R; and a gate drivingarrangement for activating the pixels of rows in dependence on a rowscanning scheme; a source driving arrangement for providing columnvoltages representing the respective image data, a number of buffersdepending on the number of grey levels for supplying the respectivevoltage levels to the columns, a switching unit for selecting a voltagelevel which corresponds to the grey level to be displayed; a controlunit for controlling the buffers, for analyzing the image data to bedisplayed, for determining a number of required buffers in dependence onthe number of grey levels to be displayed, and for switching off theunused buffers.
 2. The display device as claimed in claim 1, wherein thesource driving arrangement comprises a memory unit for storing the imagedata, said the control unit controlling the memory unit.
 3. The displaydevice as claimed in claim 2, wherein the control unit is provided foranalyzing the image data when the memory unit is written, the number ofrequired buffers is determined in dependence on the grey levels of theimage data to be displayed, and the buffers not required are switchedoff by the control unit.
 4. The display device as claimed in claim 2,wherein the control unit is provided for analyzing the image data whenthe memory unit is read out, the number of required buffers isdetermined in dependence on the grey levels of the image data to bedisplayed, and the buffers not required are switched off by the controlunit.
 5. The display device as claimed in claim 1, wherein a feedbackline between each buffer and the control unit is provided for sending afeedback from the buffer to the control unit, indicating whether therespective buffer has delivered a signal for driving a certain greylevel, and wherein the control unit is provided for analyzing thefeedback signals of the buffers during a whole frame and for switchingoff the buffers that are not required.
 6. The display device as claimedin claim 1, wherein each buffer is connected to the control unit by aswitching line.
 7. The display device as claimed claim 1, wherein thecontrol unit comprises an analyzing unit for counting the used greylevels and for determining the required buffers.
 8. The display deviceas claimed in claim 1, wherein the unused buffers are switched off untilthe memory unit is rewritten or the image to be displayed has changed.9. The display device as claimed in claim 1, wherein the buffers areconnected to the supply voltage via a resistor chain.
 10. The displaydevice as claimed in claim 1, wherein the buffers are switched off independence on an operational mode used for displaying the image data.11. A method of driving a display device having pixels arranged incolumns C and rows R, comprising the following steps: the number of greylevels used for the image to be displayed is analyzed; and depending onthe number of grey levels used, the number of buffers for providing thecolumn voltages is determined and the unused buffers are switched off.12. A method as claimed in claim 11, wherein the number of grey levelsused for the image to be displayed is analyzed each time a memory unitcontaining the image data is written.
 13. A method as claimed in claim11, wherein the number of grey levels used for the image to be displayedis analyzed each time a memory unit containing the image data is readout.
 14. A method as claimed in claim 11, wherein the number of greylevels used for the image to be displayed is analyzed by feeding back afeedback signal from the buffers, indicating whether the respectivebuffer has delivered a signal for driving a certain grey level, afterscanning of a whole frame, whereupon any unused buffers are switchedoff.
 15. A display device comprising: a liquid crystal display havingpixels which are arranged in columns and rows; a gate drivingarrangement for activating the pixels of rows in dependence on a rowscanning scheme; a source driving arrangement for providing columnvoltages representing the respective image data, the source drivingarrangement has a memory unit for storing the image data; a number ofbuffers depending on the number of grey levels for supplying therespective voltage levels to the columns; a switching unit for selectinga voltage level which corresponds to the grey level to be displayed; anda control unit for controlling the buffers and the memory unit, foranalyzing the image data to be displayed, for determining a number ofrequired buffers in dependence on the number of grey levels to bedisplayed, for switching off the unused buffers, and for analyzing theimage data when the memory unit is written, wherein, the number ofrequired buffers is determined in dependence on the grey levels of theimage data to be displayed, and the buffers not required are switchedoff by the control unit.
 16. The display device as claimed in claim 15,wherein a feedback line between each buffer and the control unit isprovided for sending a feedback from the buffer to the control unit,indicating whether the respective buffer has delivered a signal fordriving a certain grey level, and wherein the control unit is providedfor analyzing the feedback signals of the buffers during a whole frameand for switching off the buffers that are not required.